24. 
Twelfth Class. 
MOLLUSCA. 
Body sometimes naked, either destitute of any solid internal parts, or inclosing a shell or 
other hard substance, and sometimes provided externally with a shell covering or sheath- 
ing, but is never composed of two opposite valves united by a hinge. 
Tus Class is divided into Five Orders, viz. Pteropoda, Gasteropoda, Trachelipoda, 
Cephalopoda, and Heteropoda. 
ORDER I. 
PTEROPODA. —6 GENERA. 
Some only are furnished with a thin, cartilaginous or corneous shell. 
Hyat“a.—Shell corneous, transparent, ovate-globose; tridentated posteriorly; open at 
the summit, and at the two posterior sides. 
Plate XIII. Fig. 7. H. tridentata. (Monoculus telemus?—Linn.) 
Cu10o.—This genus has no shell. 
CiEeopora.—Shell gelatinous, cartilaginous, transparent, in shape of a reversed pyramid, 
or lanceolate, truncated and open at the summit. 
Plate XIII. Fig. 8. C. pyramidata. (Clio pyramidata—Linn.) 
Limacina.—Shell thin, fragile, papyraceous, spiral; the whorls re-united in a discoidal 
manner, like the Planorbis. 
Plate XIII. Fig. 9. (a) L. helicialis. (Clio helicina.—Gmel.) [British Museum. ] 
CympBuLiA.—Shell gelatinous, cartilaginous,very transparent, crystalline, oblong, in shape 
of a shoe, truncated at the summit; aperture lateral and anterior. 
Plate XIII. Fig. 10. C. Peronii. [Icon. Encye. Method. pl. 464. fig. 4 b.] 
PyeumopeRMon.—This genus has no shell. 
